In labyrinth disk stacks which are used for pressure reduction the fluid flow may be from the inside diameter of the disk stack to the outside diameter of the disk stack or from the outside to the inside. The latter is usually selected if the fluid passing through the disk is apt to be contaminated with solid particles. In this manner the solid particles too large to pass through the disk passages collect on the larger diameter where they are more easily removed and are less apt to interfere with other valve parts frequently located in the inner diameter.
Passages in the disk may be of uniform cross section area for fluids which do not expand with reducing pressure or may be designed to expand at a precise rate with each turn to control the velocity as a fluid expands with reduced pressure. In either instance design performance of the device is highly dependent on the relative cross section area of each passage and any major disturbance thereto can seriously affect the efficiency of that passage and even lead to destructive velocities, due to the fact that a reduction in passage area means higher pressure drop and higher velocity at that point.
Since the fluids that such a stack will handle often have dirt particles therein, such dirt particles tend to block or restrict the inlets to the fluid passages thereby reducing flow through the disk stack passageways and increasing the pressure drop across the inlets to the passageways. The increased pressure drop wedges the dirt particles into the inlets of the passageways as well as the passageways themselves making any cleaning operations, such as back-flushing, or other cleaning difficult if not impossible. The blocked inlets and passages are progressively rendered ineffective and the partial openings of the passageways tend to erode the passageways due to the increased velocity of the fluid flowing through the partially blocked passageways.
The placing of separate screens and filters along the periphery of the disk stack is expensive and cumbersome since it requires not only an extra part to be added to the disk stack but also adds another assembly step thereby escalating the cost of the disk stack. Also the blockage of a part of the screen renders the covered labyrinths of the stack inoperative.